This application is based on application No. JP2000-298430 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an improved driving apparatus and method for driving of a driven member, such as a disk-like rotor that rotates or a slider that moves in a linear fashion. Specifically, it relates to an improved driving apparatus and method using electromechanical conversion elements, and more specifically to a truss-type improved driving apparatus and method using electromechanical conversion elements. More particularly, it relates to a driving apparatus and a driving method for electromechanical conversion elements belonging to an ultrasonic motor or similar mechanism that uses drive signals that have been subjected to frequency modulation using as the central frequency a frequency near the mechanical resonance frequency of the electromechanical conversion element.
2. Description of the Related Art
As a type of ultrasonic motor mentioned above, the motor having the construction shown in FIG. 7 is known. This ultrasonic motor is a truss-type actuator comprising a driving unit 1 and a pressure unit 6. The driving unit 1 comprises two displacement members 2 and 3 that cross each other at a prescribed angle (for example, 90xc2x0), a synthesizing member 5 that is made to adhere at the crossing point thereof and a fixing member 4 that is made to adhere to the ends thereof. The pressure unit 6 comprises a coil spring or similar mechanism and presses the fixing member 4 toward the center of the rotor (i.e., in the direction of the arrow A), which comprises a driven member 7.
This ultrasonic motor may be driven based on non-resonance driving, but if resonance driving is used, efficient low-voltage driving is enabled. Therefore, in general, an ultrasonic motor is driven in a resonance mode.
As a driving apparatus that performs resonance driving of an ultrasonic motor, the apparatus shown in FIG. 8 is proposed (Japanese Laid-Open Patent Application 2001-16879, etc.).
This driving apparatus includes two power amplifiers 102 and 103 that amplify the drive signals from the oscillator 101 and drive the displacement members 2 and 3, respectively, a phase converter 104 that is placed between the oscillator 101 and the power amplifier 103, current detectors 105 and 106 that detect the current values drawn to the displacement members 2 and 3 using resistors R, respectively, and a phase difference detector 107 that detects the phase difference based on the output signals from the current detectors 105 and 106. It further includes a resonance frequency detector 108, two switches 109 and 110, and an MPU 111. The switch 109 has two ON/OFF switch members 109a and 109b. The switch 110 has two ON/OFF switch members 110a and 110b. When the ON/OFF switch members 109a and 110a are both ON, the resonance frequency detector 108 detects the phase difference between the output signal (voltage component) of the oscillator 101 and the output signal (current component) of the current detector 105, and where the driving frequency of the oscillator 101 is higher than the resonance frequency that enables resonance driving (at which there should be no phase difference), the resonance frequency detector 108 outputs to the MPU 111 a signal that reduces the driving frequency of the oscillator 101, while where the driving frequency of the oscillator 101 is lower than the resonance frequency, it outputs to the MPU 111 a signal that increases the driving frequency of the oscillator 101. On the other hand, where the ON/OFF switch members 109b and 110b are ON, the resonance frequency detector 108 detects the phase difference based on the output signal (voltage component) of the phase converter 104 and the output signal (current component) of the current detector 106, and where the driving frequency of the oscillator 101 is higher than the resonance frequency of the displacement member, the resonance frequency detector 108 outputs to the MPU 111 a signal that reduces the driving frequency of the oscillator 101, while where the driving frequency of the oscillator 101 is lower than the resonance frequency, it outputs to the MPU 111 a signal that increases the driving frequency of the oscillator 101.
The MPU 111 reduces or increases the driving frequency of the oscillator 101 based on the signal from the resonance frequency detector 108 in order to adjust the driving frequency of the oscillator 101 to a frequency that enables resonance driving. The elliptical locus of the synthesizing member of this driving apparatus depends on the phase difference of the displacement of each displacement member.
In the driving apparatus described above, drive signals are supplied to the two displacement members and both members are driven, but in addition to this scenario, it is also possible to supply a drive signal only to one of the displacement members such that only one displacement member is driven. A method for this type of one displacement member driving has also been proposed.
FIG. 9 is a block diagram showing the proposed one displacement member driving apparatus.
This driving apparatus amplifies the output signal of the oscillator 201 using the power amplifier 202, and the amplified signal therefrom is supplied to the displacement member 2 or 3 via the switch 203. At the same time, the currents drawn to the displacement members 2 and 3 are detected by separate current detectors 205 and 206, respectively, and based on the current phase difference between these two currents, the MPU 211 adjusts the oscillation frequency of the oscillator 201. The locus of the synthesizing member changes depending on the driving frequency.
Incidentally, using the above driving apparatus that drives both displacement members, the resonance frequency of the displacement member changes as the load and/or environment fluctuate, and if the driving frequency is offset from the resonance frequency, the driving characteristics change substantially. Depending on the degree of such offset, the driving apparatus may not be operated or may stop. Therefore, where an ultrasonic motor is driven based on resonance driving, the oscillation status must be fed back to ensure that the driving frequency matches the resonance frequency of each element.
In addition, in the case of the driving apparatus that drives only one displacement member, because locus control is performed by changing the driving frequency based on the phase difference of the currents that are drawn to the elements, the oscillation status of the displacement member must be fed back.
Where the oscillation status of the displacement member is fed back, a feedback circuit is needed.
The present invention was created in view of this situation, and an object thereof is to provide an improved driving apparatus and method for an actuator. Specifically, an object of the present invention is to improve a truss-type driving apparatus and method using electromechanical conversion elements, and more particularly, to provide a driving apparatus that drives electromechanical conversion elements without feedback.
In order to attain this and other objects, according to one aspect of the present invention, the driving apparatus has (i) a base, (ii) multiple displacement members, the base ends of which are fixed to the base and the tip ends of which are combined at one location, said displacement members each generating a prescribed displacement, (iii) a pressure unit that keeps the synthesizing member, at which the tip ends of the displacement members are combined, in pressure contact with the driven member, which comprises the object of driving, (iv) a drive circuit that impresses drive signals to the displacement members, and (v) a controller that controls the drive circuit so that the synthesizing member moves in an elliptical path and the drive force is transmitted to the driven member, wherein the controller drives at least one of the multiple displacement members using a drive signal that has been subjected to frequency modulation.
According to this aspect of the invention, because the displacement member or elements are driven using a drive signal or signals that have been subjected to frequency modulation, it is no longer necessary to match the driving frequency to the resonance frequency regardless whether resonance or non-resonance driving is performed, and the oscillation status of the displacement members need not be fed back. As a result, the circuitry may be simplified and the number of components may be reduced.
In the driving apparatus according to another aspect of the invention, the central frequency of the drive signal that has been subjected to frequency modulation essentially matches the resonance frequency of the displacement member. In addition, the modulation difference of the drive signal is larger than the range of change in the resonance frequency of the displacement member that occurs due to fluctuations in the ambient environment of the displacement member. Moreover, the modulation difference of the drive signal is larger than the range of change in the resonance frequency of the displacement member that occurs due to fluctuations in the load placed on the displacement member.
According to this aspect of the invention, because the drive signal uses as its central frequency a frequency near the resonance frequency, the actuator may be driven based on resonance driving. In addition, because the modulation difference of the drive signal is set to be larger than the range of change in the resonance frequency that occurs due to fluctuations in the load or the environment, even when the resonance frequency changes due to fluctuations in the load or the environment, driving may be performed regardless of such fluctuations.
In the driving apparatus according to another aspect of the invention, the controller controls the speed of the motion of the driven member by changing at least one of (i) the central frequency, (ii) the modulation difference, or (iii) the modulated frequency of the frequency-modulated signal, which is the drive signal.
According to this aspect of the invention, if at least one of the central frequency, the modulation difference or the modulated frequency of the frequency-modulated signal is changed, the proportion of the higher-than-resonance frequency range in the frequency-modulated signal changes, through which the speed of the driven member is controlled. Consequently, low-speed driving, at which an actuator is not particularly effective, is enabled.
According to another aspect of the present invention, the driving apparatus has (i) a base, (ii) multiple displacement members, the base ends of which are fixed to the base and the tip ends of which are combined at one location, said displacement members each generating a prescribed displacement, (iii) a pressure unit that keeps the synthesizing member, at which the tip ends of the displacement members are combined, in pressure contact with the driven member, which comprises the object of driving, (iv) a drive circuit that impresses a drive signal to one of the displacement members, and (v) a controller that causes the synthesizing member to perform an elliptical motion such that the driving force is transmitted to the driven member. Such motion is made to occur through control of the drive circuit such that one of the multiple displacement members is driven using a drive signal that has been subjected to frequency modulation, wherein the controller impresses to the displacement member to be driven a drive signal that has been subjected to frequency modulation and uses as its central frequency a drive frequency at which the current phase difference between the driven displacement member and the following displacement member, which is not directly driven, becomes approximately 90 degrees.
Using this driving apparatus as an actuator, an area exists in which the driven member moves at an essentially constant speed, as shown by the black dots in FIG. 6 (i.e., the area between approximately 79 kHz and 95 kHz), and the drive-enabled area is large. As a result, driving at an essentially constant speed is possible without feedback of the oscillation status of the displacement members.